The present invention relates to solid state slab lasers otherwise known as total internal reflection, face pumped lasers (TIR-FPL), and more particularly to laser devices having improved output power and beam quality performance.
A total internal reflection face pumped laser comprises a solid state lasing medium such as neodymium yttrium aluminum garnet or neodymium glass in the form of a flat generally rectangular slab. The slab is optically pumped by lamps or other sources in such a way that electromagnetic energy impinges on the large planar surfaces of the lasing medium to pump the atoms in the medium to an elevated metastable energy state.
During operation of the laser, considerable heat is generated within the lasing medium in response to optical pumping to produce a population inversion of atoms. Surface cooling, as by passing a fluid coolant over the large planar faces of the slab, for example, is generally employed to remove heat from the lasing medium. Most solid state laser materials, however, have poor thermal conductivity, and surface cooling results in a thermal gradient between the cooled outer surface and the relatively hot center region of the laser slab. This produces a variable thermal strain in the slab, caused by the center of the slab being in compression and the relatively cool surface being in tension. Since the index of refraction of the lasing medium is a function of both temperature and stress, solid state lasers surfer from thermally induced beam defocusing, birefringence and depolarization. Because of thermal gradients, slab lasers typically exhibit both width-wise and thickness-wise wavefront distortions of the laser beam. The thickness-wise distortions are acceptable as long as they are symmetrical with respect to the longitudinal axis. The width-wise distortions caused by the width-wise or transverse temperature gradients are not acceptable. These are most prominent near the lateral side surfaces or faces of the slab, and the optical distortion is most pronounced in such areas. If the variable optical distortion of the beam is large enough, it may not be possible to compensate it with a single lens. In practice, this results in a decrease in the power output of the laser for a given beam quality, where beam quality is defined as the product of the output beam diameter and its angular divergence. With typical slabs having an aspect ratio (slab width to slab thickness) in the range of 2 to 2.5, the region of uniform optical distortion may be limited to about only the central one-third of the slab. While it is acceptable for the temperature of the slab to vary symmetrically across the slab thickness (the small dimension) the temperature should be uniform and should not vary across the major planar surface or face of the slab transverse to the direction of slab thickness.
Attempts to reduce temperature gradients in slab lasers have included the use of siderails in order to control the transverse flow of heat through the lateral faces of the slab. Siderails function as thermal insulators to prevent the transverse flow of heat across the lateral faces, and they assist in achieving a uniform temperature distortion across the major surface of the slab. Siderails, however, have proved to be less than satisfactory.
Although siderails may be good barriers to thermal conduction, they may easily transmit optical and thermal radiation from the optical pumping sources resulting in a high level of slab heating adjacent to the siderails. Consequently, the optical distortion in these regions differs from that in the center of the slab, and this causes decreased output power/beam quality performance.
In order to counteract this effect, partial siderails, i.e., siderails having a height which is less than the thickness of the slab, have been proposed. This enables coolant fluid to contact the exposed portions of the side surfaces of the slab to reduce local heating. Although such partial siderails may afford some uniformity in the transverse thermal distribution and some reduction in optical distortion in the slab, it is difficult to match the proper size of the siderail to the laser, and the siderail height must often be found by trial and error. The siderail must also be tailored to each laser. Moreover, since the temperature gradients in the laser slab are a function of many different variables, a given partial siderail may be effective only over a limited range of optical input power and coolant flow rates.
There is a need for a better way of controlling the transverse temperature gradients in TIR-FPL slab devices which can accommodate a side variation in thermal environments and operating conditions and which may be adjusted or tuned on line to match a particular set of conditions to afford optimum laser performance. It is to these ends that the present invention is directed.